Add the vhost-user netdev backend to the command line

The supplied chardev id will be inspected for supported options. Only
a socket backend, with a set path (i.e. a Unix socket) and optionally
the server parameter set, will be allowed. Other options (nowait, telnet)
will make the chardev unusable and the netdev will not be initialised.

Additional checks for validity:
  - requires `-numa node,memdev=..`
  - requires `-device virtio-net-*`

The `vhostforce` option is used to force vhost-net when we deal with
non-MSIX guests.

Signed-off-by: Antonios Motakis <a.motakis@virtualopensystems.com>
Signed-off-by: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

qemu-options.hx

@@ -1460,6 +1460,7 @@ DEF("netdev", HAS_ARG, QEMU_OPTION_netdev,
 #ifdef CONFIG_NETMAP
     "netmap|"
 #endif
+    "vhost-user|"
     "socket|"
     "hubport],id=str[,option][,option][,...]\n", QEMU_ARCH_ALL)
 STEXI
@@ -1791,6 +1792,23 @@ The hubport netdev lets you connect a NIC to a QEMU "vlan" instead of a single
 netdev.  @code{-net} and @code{-device} with parameter @option{vlan} create the
 required hub automatically.
 
+@item -netdev vhost-user,chardev=@var{id}[,vhostforce=on|off]
+
+Establish a vhost-user netdev, backed by a chardev @var{id}. The chardev should
+be a unix domain socket backed one. The vhost-user uses a specifically defined
+protocol to pass vhost ioctl replacement messages to an application on the other
+end of the socket. On non-MSIX guests, the feature can be forced with
+@var{vhostforce}.
+
+Example:
+@example
+qemu -m 512 -object memory-backend-file,id=mem,size=512M,mem-path=/hugetlbfs,share=on \
+     -numa node,memdev=mem \
+     -chardev socket,path=/path/to/socket \
+     -netdev type=vhost-user,id=net0,chardev=chr0 \
+     -device virtio-net-pci,netdev=net0
+@end example
+
 @item -net dump[,vlan=@var{n}][,file=@var{file}][,len=@var{len}]
 Dump network traffic on VLAN @var{n} to file @var{file} (@file{qemu-vlan0.pcap} by default).
 At most @var{len} bytes (64k by default) per packet are stored. The file format is